Ethylenimine is a potential chemical intermediate for the production of linear ethyleneamine polymers and for the production of various other amines. However, because of the toxicity and carcinogenicity of ethylenimine, this material poses severe handling difficulties, which make it highly undesirable to store or transport the ethylenimine, so that desirably a process for the production of ethylenimine should begin from inexpensive starting materials, and should provide the ethylenimine in a form which permits its direct feed to the ethyleneamine production unit without intervening isolation or storage of the ethylenimine.
Various processes for the production of ethylenimine are known. For example, ethylenimine may be produced by the reaction of ethylene dichloride with anhydrous ammonia. However, this method suffers from the disadvantages of involving halide use and producing a salt by-product.
One commercially attractive process for the production of ethylenimine is the catalytic dehydration/deamination of monoethanolamine. Monoethanolamine is known to convert to a mixture of various alkanolamines, alkylamines and aziridines when contacted with various heterogeneous catalysts. Similar mixtures are obtained from other alkanolamines using the same catalysts. However, typically such reactions produce a complex mixture of products.
Most of the known catalysts for these dehydration/deamination reactions of alkanolamines are oxides of tungsten, tantalum or niobium, in some cases promoted with transition metals such as iron or chromium, the metal oxides usually being disposed upon a support, for example silica or alumina.
For example, U.S. Pat. No. 4,289,656, issued Sept. 15, 1981, and U.S. Pat. No. 4,358,405, both to Hayes et al., describe a dehydration catalyst and process for making an alkylenaziridine (such as ethylenimine) from an alkanolamine (such as monoethanolamine); the catalyst contains oxides of either tantalum or niobium together with the oxides of iron and chromium, in which the ratios of the metals are: EQU M.sub.10 Fe.sub.0.5-2.9 Cr.sub.0.3-1.7
wherein M is tantalum or niobium.
U.S. Pat. No. 4,301,036, issued Nov. 17, 1981 to Childress et al., describes a dehydration catalyst for the dehydration of alkanolamines to alkylenaziridines. This dehydration catalyst is prepared by applying a solution of a tungsten salt on to a low surface area support (usually silicon carbide), calcining the salt to tungsten oxide, and thereafter applying silica to the tungsten-coated support so as to form a coating of silica over the tungsten.
U.S. Pat. No. 4,337,175, issued June 29, 1982 to Ramirez, describes a dehydration catalyst for the dehydration of alkanolamines to alkylenaziridines. This dehydration catalyst consists essentially of an oxide of tantalum or niobium with an alkaline earth metal oxide as a promoter on an inert support, for example a low surface area, high purity alumina.
Other catalysts have been used for the production and conversion of monoalkanolamines. For example, U.S. Pat. No. 4,524,143, issued June 18, 1985 to Vanderpool, describes a process for the production of linear polyethylenepolyamines from ethylenediamine and monoethano amine using thermally activated pelleted catalyst compositions comprising zirconium silicate having phosphorus deposited thereon.
The relatively complex mixtures produced by these prior art processes pose obvious problems of separation. In addition, the relative proportions in which the various products are produced are rarely optimal with regard to the commercial demand for, and selling prices of, the various products, and thus it would be highly advantageous to be able to vary the product distribution to increase the proportions of the more valuable products produced.
Because of their microporous structure, with pores of uniform size, molecular sieves offer the possibility of modifying the product distribution obtained in the aforementioned dehydration/deamination reactions of alkanolamines. However, the present inventors are aware of only one patent (U.S. Pat. No. 3,956,329) describing the use of molecular sieves in these reactions, and in this patent the molecular sieves used are conventional zeolite aluminosilicates produced by direct synthesis and having silicon:aluminum ratios less then 6.
It has now been discovered that the product distribution obtained in the aforementioned dehydration/deamination reactions of alkanolamines can be improved by using as the catalyst in such reactions certain selected molecular sieves.